Conventionally, there is well known an end mill that is a tool attached to a milling machine, a machining centre or the like so as to process (cut) an object by touching a tip of the ball end mill on which cutting edges are formed with the object and rotating and moving along the direction perpendicular to the spindle.
With regard to the end mill, a ball end mill that cutting edges are formed on a hemispheric tip is largely used for three-dimensional processing, for example processing of a cavity face of a mold having a complicated curved surface.
By the cutting processing with the ball end mill, an area with low surface accuracy (with unevenness) referred to as “pluck” may be formed on a work face of an object.
Especially as shown in FIGS. 10(a) and (b), the smaller the work face angle α, which is an angle of the work face about a surface substantially perpendicular to the spindle of the ball end mill, becomes, the more the face roughness increases. By observing the work face with small work face angle α and large face roughness, irregular unevenness, that is, “pluck” is found to occur on the work face. FIG. 10(b) shows an example of processing with a ball end mill whose diameter is 20 mm.
When the “pluck” occurs on the work face, processing such as grinding is required separately so as to secure desired face accuracy, whereby the problem occurs that the number of man-hour for the processing is increased.
An area of a point on the spindle (what is called a NULL point) and the vicinity thereof in the tip of the ball end mill are dragged while touching the work face so that the “pluck” occurs on the work face. By detailed analysis, the mechanism of the occurrence can be classified broadly into four, (1) circumferential speed zero pluck, (2) low circumferential speed pluck, (3) flank pluck, and (4) chip catching pluck.
Explanation will be given on the mechanism of the occurrence of each kind of the “pluck” referring a conventional ball end mill 100 shown in FIG. 11.
The ball end mill 100 includes a substantially cylindrical member, and a basal part which is one of ends thereof is detachably fixed to a rotary drive shaft of a milling machine, a machining centre or the like. A tip which is the other end of the ball end mill 100 is hemispheric, and cutting edges 101 and 102 are formed on the tip.
The cutting edges 101 and 102 are arranged at the positions respectively rotated for 180° from each other about the spindle of the ball end mill 100 when viewed along the axial direction of the spindle.
The cutting edge 101 has a rake face 101a which is a face taking part in the cutting and a flank 101b which is a face not taking part in the cutting, and the border between the rake face 101a and the flank 101b is a ridgeline 101c. The rake face 101a is parallel or nearly parallel to the spindle of the ball end mill 100. The flank 101b is perpendicular or nearly perpendicular to the spindle of the ball end mill 100.
When viewed along the axial direction of the spindle of the ball end mill 100, one of the ends of the ridgeline 101c of the rake face 101a is in agreement with a NULL point 105 of the ball end mill 100, and the ridgeline 101c of the rake face 101a is extended along the radial direction of the ball end mill 100.
Similarly, the cutting edge 102 has a rake face 102a which is a face taking part in the cutting and a flank 102b which is a face not taking part in the cutting, and the border between the rake face 102a and the flank 102b is a ridgeline 102c. The rake face 102a is parallel or nearly parallel to the spindle of the ball end mill 100. The flank 102b is perpendicular or nearly perpendicular to the spindle of the ball end mill 100.
When viewed along the axial direction of the spindle of the ball end mill 100, one of the ends of the ridgeline 102c of the rake face 102a is in agreement with the NULL point 105 of the ball end mill 100, and the ridgeline 102c of the rake face 102a is extended along the radial direction of the ball end mill 100.
The NULL point 105 is a point on the spindle of the ball end mill 100 in the tip of the ball end mill 100 (the intersection point of the tip of the ball end mill 100 and the spindle of the ball end mill 100).
Accordingly, the inner circumference end of each of the rake face 101a of the cutting edge 101 and the rake face 102a of the cutting edge 102 of the ball end mill 100 is not offset about the rotation direction, and the inner circumference end is arranged on the spindle of the ball end mill 100.
For performing the cutting processing with the ball end mill 100 without causing the “pluck” on the rake face, it is important to keep circumferential speed V of the cutting edges 101 and 102 at the touching part of the ball end mill 100 and the object not less than a predetermined “threshold” so that the cutting edges 101 and 102 shear the unprocessed portion of the object.
Using distance r between the ball end mill 100 and the spindle and rotation speed S of the ball end mill 100, the circumferential speed V is indicated with the relation V=2π*r*S.
However, since the NULL point 105 is positioned on the spindle of the ball end mill 100 (r=0), the circumferential speed V at the NULL point 105 is zero, whereby the cutting edges 101 and 102 do not shear the unprocessed portion of the object.
Then, the NULL point 105 is dragged on the work face substantially at feed speed F of the ball end mill 100 (movement speed of the ball end mill 100 along the direction substantially perpendicular to the spindle) which is smaller than the “threshold”.
The “pluck” caused at the touching part of the NULL point 105 and the work face with the mechanism as mentioned above is the “circumferential speed zero pluck”.
At the area in the vicinity of the NULL point 105, that is, at the end at the side of the NULL point of the cutting edges 101 and 102, the distance r from the spindle is small so that the circumferential speed V is also small, whereby the shearing of the unprocessed portion of the object with the cutting edges 101 and 102 is weak.
Then, the area in the vicinity of the NULL point 105 is dragged on the work face at the speed substantially the same as the feed speed F of the ball end mill 100 (movement speed of the ball end mill 100 along the direction substantially perpendicular to the spindle) which is smaller than the “threshold”.
The “pluck” caused at the touching part of the area in the vicinity of the NULL point 105 and the work face with the mechanism as mentioned above is the “low circumferential speed pluck”.
With regard to the area in the vicinity of the NULL point 105, especially at the area at which the feed speed F is larger than the circumferential speed V (the area at which V<F is satisfied), the rake face 101a of the cutting edge 101 (the rake face 102a of the cutting edge 102) pushes out (deforms plastically) the unprocessed portion of the object along the feed direction of the ball end mill 100 when the rake face 101a (the rake face 102a) is disposed toward the feed direction.
Then, the area at which the feed speed F is larger than the circumferential speed V with regard to the area in the vicinity of the NULL point 105 is dragged on the work face while the unprocessed portion of the object is pushed out along the feed direction.
The “pluck” caused at the area at which the feed speed F is larger than the circumferential speed V with regard to the area in the vicinity of the NULL point 105 with the mechanism as mentioned above is the “flank pluck”.
With regard to the chips (scraps) generated by the cutting processing with the ball end mill 100, the chips generated in the vicinity of the NULL point 105 are not discharged efficiently to the outside because the circumferential speed in the vicinity of the NULL point 105 is small. The chips may be dragged while pinched between the tip of the ball end mill 100 (the area in the vicinity of the NULL point 105) and the work face of the object, or may be dragged while the surfaces of the chips are melted by the frictional heat and the chips adhere (are welded) to the tip of the ball end mill 100. Flaws (unevenness) generated on the work face by the drag of the chips pinched between the work face and the tip of the ball end mill 100 or the chips welded to the tip of the ball end mill 100 as mentioned above is the “chip catching pluck”.
Such “chip catching pluck” tends to be caused especially in the case that the object of the processing (cut object) is a cast material such as cast iron. That is because the cast material generally has low hardness so that the cutting edge in the vicinity of the NULL point can cut into the work face easily and the size of the chips (scraps) generated by the processing is small.
As mentioned above, when the object is processed with the conventional ball end mill 100, various kinds of “pluck” such as (1) circumferential speed zero pluck, (2) low circumferential speed pluck, (3) flank pluck, and (4) chip catching pluck are caused.
Each of the chips in the case that such “pluck” is caused has characteristics that the shape is not uniform, the surface is lusterless (husky), the tip is round, refuse (minute comminuted chips) is included, and the like.
Conventionally, various kinds of ball end mills having tips characteristic in shape are proposed. Examples thereof are described in the Patent Literatures 1 to 15.
In the ball end mill described in the Patent Literature 1, the rake face of the cutting edge is offset forward in the rotation direction of the ball end mill (the blade is below the center of the object).
The ball end mill described in the Patent Literature 1 is advantageous in abrasion resistance and shock resistance.
However, the ball end mill described in the Patent Literature 1 has a problem that, though the inner circumference end of the rake face of the cutting edge is shifted along the offset direction from the NULL point, the occurrence of (2) low circumferential speed pluck cannot be prevented within the range of offset amount for making the dimension error of the work face within predetermined tolerance.
In each of the ball end mills described in the Patent Literatures 2 to 4, two cutting edges are arranged so that the phases thereof are shifted from each other for 180°. The inner circumference end of the rake face of one of the cutting edges protrudes toward the opposite side (the side at which the other cutting edge is formed) astride the spindle, and the inner circumference end of the rake face of the other cutting edge does not protrude astride the spindle and is arranged at a position evacuated from the spindle for predetermined distance.
The ball end mill described in the Patent Literature 2 is advantageous that the chipping at the inner circumference end of the cutting edge is prevented.
However, each of the ball end mills described in the Patent Literature 2 to 4 has a problem that the flank of the cutting edge protruding toward the opposite side astride the spindle is positioned at the NULL point so that the occurrence of (1) circumferential speed zero pluck cannot be prevented.
The ball end mills described in the Patent Literatures 2 to 4 have a problem that the occurrence of (3) flank pluck cannot be prevented by the flank of the cutting edge protruding toward the opposite side astride the spindle.
In each of the ball end mills described in the Patent Literatures 5 to 8, a recess (relief) is formed at the area of the NULL point and the vicinity thereof.
Each of the ball end mills described in the Patent Literatures 5 to 8 is advantageous because of not touching the work face so as to prevent the occurrence of (1) circumferential speed zero pluck.
However, each of the ball end mills described in the Patent Literatures 5 to 8 has a problem that the occurrence of (2) low circumferential speed pluck cannot be prevented. That is because of below reason.
Each of the ball end mills described in the Patent Literatures 5 to 8 is constructed so that the rake face of the other cutting edge is not offset about the rotation direction of the ball end mill, whereby the distance between the inner circumference end of the rake face of the cutting edge and the NULL point is the same as the radius of the recess. For making the dimension error of the work face within the predetermined tolerance, the radius of the recess cannot be very large, whereby the circumferential speed of the inner circumference end of the rake face of the cutting edge is small and (2) low circumferential speed pluck occurs.
In the ball end mill described in the Patent Literature 9, the rake face of the cutting edge is offset forward in the rotation direction of the ball end mill (the blade is above the center of the object).
However, in the ball end mill described in the Patent Literature 9, the inner circumference ends of the rake faces of a pair of the cutting edges are shifted along the offset direction of the rake faces of the cutting edges from the NULL point, and the connecting portion between the flanks of the pair of the cutting edges is disposed at the NULL point, whereby the connecting portion is dragged while touching the work face. Accordingly, there is a problem that the occurrence of (2) low circumferential speed pluck cannot be prevented.
The ball end mill described in the Patent Literature 9 has a problem that the offset amount of the rake faces of the cutting edges is small and the circumferential speed of the inner circumference ends of the rake faces of the pair of the cutting edges is small so that the occurrence of (2) low circumferential speed pluck cannot be prevented.
In each of the ball end mills described in the Patent Literatures 10 to 13, the inner circumference ends of a plurality of the cutting edges formed at the tip are connected by a chisel blade formed astride the NULL point, and the work face touching the area of the NULL point and the vicinity thereof is burnishing-processed with the chisel blade.
However, each of the ball end mills described in the Patent Literatures 10 to 13 has a problem that the burnishing processing is originally performed by dragging the chisel blade while the chisel blade touches the work face so that the occurrence of (1) circumferential speed zero pluck cannot be prevented.
There is also a problem that the chisel blade is short in the radial direction so that the occurrence of (2) low circumferential speed pluck and (3) flank pluck cannot be prevented.
Furthermore, there is a problem that large load is applied on the chisel blade so that the chipping tends to be caused.
The ball end mill described in the Patent Literature 14 is constructed so that the chisel blade of the ball end mill described in the Patent Literature 10 is extended in the radial direction.
Since the chisel blade has the cutting faces at both side faces and has no flank, compared with the ball end mill described in the Patent Literature 10, the ball end mill described in the Patent Literature 14 is advantageous that the occurrence of (3) flank pluck can be prevented.
However, the ball end mill described in the Patent Literature 14 has a problem that the angle between the cutting face of the chisel blade and the work face is small so that the cutting ability is lower than the normal cutting edge that the angle between the cutting face of the chisel blade and the work face is about 90°.
Even if the chisel blade is extended in the radial direction, the problem that the occurrence of (1) circumferential speed zero pluck cannot be prevented is not solved.
In the ball end mill described in the Patent Literature 15, one chisel blade is arranged at the position offset from the NULL point.
The ball end mill described in the Patent Literature 15 is advantageous that the chisel blade is arranged at the position offset from the NULL point so that the occurrence of (1) circumferential speed zero pluck can be prevented and both side faces of the chisel blade are the cutting faces so that the occurrence of (3) flank pluck can be prevented.
However, the ball end mill described in the Patent Literature 15 has a problem that the distance between the substantially center of the edge of the chisel blade and the NULL point so that the occurrence of (2) low circumferential speed pluck cannot be prevented.
Each of the ball end mills described in the Patent Literatures 1 to 15 is originally designed not for preventing the occurrence of (4) chip catching pluck.
Especially each of the ball end mills described in the Patent Literatures 10 to 15 has a problem that the rake face of the chisel blade is widely slanted from the position perpendicular to the work face so as to tend to crush the chips entering the gap between the chisel blade and the work face and to cause the welding, whereby (4) chip catching pluck tends to be caused.
The “pluck” is caused when the area of the NULL point and the vicinity thereof of the ball end mill touches the work face. Then, as another method for preventing the occurrence of the “pluck”, it is conceivable that the angle of the fixation support part for the ball end mill in a milling machine or a machining centre is changeable so as to prevent the area of the NULL point and the vicinity thereof of the ball end mill from touching the work face always at the time of processing.
However, the method has problems that the construction of the milling machine or the machining centre is complicated so as to increase the cost of equipment and, in the case that the angle of the fixation support part for the ball end mill of the ball end mill is changeable, clearance (backlash) between members constituting the mechanism for the change of angle may cause the dimensional accuracy of the work face, and the like.    [Patent Literature 1] the Japanese Utility Model Laid Open Gazette Sho. 63-189518    [Patent Literature 2] JP H5-138425 A    [Patent Literature 3] JP H5-228714 A    [Patent Literature 4] JP H10-249623 A    [Patent Literature 5] JP H7-1218 A    [Patent Literature 6] JP H9-262713 A    [Patent Literature 7] JP H9-267211 A    [Patent Literature 8] JP 2000-52125 A    [Patent Literature 9] JP 2001-1208 A    [Patent Literature 10] JP 2001-293609 A    [Patent Literature 11] JP 2001-341026 A    [Patent Literature 12] JP 2002-254234 A    [Patent Literature 13] JP 2003-53617 A    [Patent Literature 14] JP 2004-181563 A    [Patent Literature 15] JP 2004-142055 A